Process for prilling urea

ABSTRACT

UREA IS PRILLED BY SPRAYING UREA DROPLETS DOWNWARDLY INTO A SOLIDIFICATION ZONE WHERE IT IS CONTACTED WITH A SUBSTANTIALLY INERT GAS. PRILLS ARE DIRECTED INTO A COLLECTOR BY DIVERTING THEIR FLOW BY DIRECTING A GAS THROUGH A PERFORATED, INVERTED CONE OR PYRAMID LOCATED AT THE BOTTOM OF AND IN COMMUNICATION WITH SAID ZONE.

Sept l7, 1974 1. MAvRovlc 3,836,611

PROCESS FOR PRILLING UREA Filed May` 22. 1972 Air Montfen UreaFf'DrQplSfs [lll/Ill] lll/lll. A r

United States Patent O U.S. Cl. 264-14 4 Claims ABSTRACT OF THEDISCLOSURE Urea is prilled by spraying urea droplets downwardly into asolidication zone where it is contacted with a substantially inert gas.Prills are directed into a collector by diverting their ow by directinga gas through a perforated, inverted cone or pyramid located at thebottom of and in communication with said zone.

This application is a continuation-in-part of application Ser. No.104,998, filed Jan. 8, 1971 as is related pending application Ser. No.255,726, filed May 22, 1972. Application Ser. No. 104,998 has beenabandoned.

FIELD OF INVENTION This invention is related to the processing of anaqueous urea solution to anhydrous solid urea of the shape of smallspherical particles usually called urea prills.

BACKGROUND OF THE INVENTION Urea is commercially obtained by reactingNH3 and CO2 at elevated pressure and temperature to form arnmoniumcarbamate and to simultaneously dehydrate ammonium carbamate to urea.The aqueous urea solution produced in a urea synthesis plant usuallycontains about one mole of water per mole of urea formed in the ureasynthesis reactor. The aqueous urea product solution generally 75-76weight percent) is usually processed to a solid anhydrous form by meansof the following sequential steps:

1. Practically all of the water contained in the aqueous urea productsolution is evaporated to form a more or less pure urea melt;

2. The resulting urea melt at about 270-300 F. is nely divided intosmall droplets either by means of a spray head, a spinning conicalbasket, or a vibrating plate, such devices usually being located at thetop of a tall, vertical, cylindrical, rectangular or square tower100-150 feet high;

3. The molten urea -droplets are allowed to fall freely inside the towercountercurrently to an uprising stream of ambient air;

4. The finely divided urea droplets in their free fall inside the towerare cooled by the uprising stream of ambient air and they are frozeninto the shape of small spherical particles (prills);

5. The frozen prills are usually cooled further to about 10U-150 F. andare collected at the bottom of the tower.

Alternate techniques can be used to produce molten urea Lin step ldescribed above. One is the crystal-melting technique, which consists ofcrystallizing the aqueous urea product solution to produce pure crystalurea, which iS washed, dried and remelted to produce a substantiallypure urea melt.

Referring to the step 5 described above, various methods of collectingsolid urea prills at the bottom of the prilling tower have been proposedaccording to the prior art. In general these methods, mainly listedbelow, have certain specific disadvantages.

3 ,83 6,61 l Patented Sept. 17, 1974 One method consists of collectingthe solid urea prills by means of a prilling tower bottom of the shapeof a truncated inverted cone or pyramid, with steep, flat sides with anopening at the bottom apex of the inverted cone, through which theprilled urea product is withdrawn. This type of prill collecting bottomhas the drawback of frequent solid urea buildup on the conical sides andon the bottom section of the inverted cone, with consequent plant shutdowns for cleaning operation. Another drawback of this type of prillcollecting bottom is the fact that relatively taller and thus moreexpensive prilling towers are required due to the very steep angle withthe horizontal, 60 or more, at which the sides of the inverted cone mustbe designed in order to facilitate the sliding of the solid prilled ureaproduct towards the bottom opening.

Still another commonly used method consists of collecting the solidprilled urea product by means of a flat horizontal prill tower bottom,which is provided with a collecting rake travelling in a circular motionand thus pushing the solid prilled urea product toward the center of thehorizontal prill tower bottom for discharge onto a belt conveyor. Such aprilling tower usually has the drawback of solid urea build up on theprilling tower bottom and on the collecting rake, and the drawback of arelatively higher product degradation due to attrition with a consequentgreater air pollution problem in the subsequent solid handling steps.

A third method commonly used in the industry consists of collecting thesolid prilled urea product by means of a horizontal uidized bed of solidurea prills of several inches in depth, which is maintained iluidized byblowing a substantial amount of air through a perforated horizontalmetallic surface upwardly into the prilling tower. The excess solidprilled urea product collected in the bottom iluidized bed is overowedfrom the fluidized bed over a weir and into a collecting trough. Such amethod has the drawback of the instability of the operation due to thecollapsing of the uidized bed of urea prills at the slightest variationin air ow through lthe perforated horizontal surface. Another drawbackof this method is the relatively high electrical power consumption bythe large air blowers required to maintain the bed of solid urea prillsproperly uidized throughout the full horizontal cross sectional area ofthe prilling tower bottom section.

Por example, in U.S. Pat. No. 3,615,142. (Dahlbom), a prilling towerwith an inverted frustum trough bottom exit is described. A louverconstruction is employed in the trough for the purpose 0f air beingpassed through the louvers to cool the prills falling through the tower.This construction has the disadvantage of olering relatively largesurface areas of the louvers on which prills can collect. With alouvered construction, there is a substantial build up of prills uponthe individual louvers and large dead airv areas. With this build upH ofprills, it is diflicult to remove the prills fro'rn the bottom sectionof the tower. Withjexcessive prill build upon the louvers, air ilow isseriously reduced; then, hot soft prills fall to the bottom andaggregate and lcause physical-failure of the louvers. Further, the airis not finely dispersedpassing through the louvers.

In U.S. Pat. No. 3,457,336 (Harris), urea orv other.

dropletsofwmoltenl material are passed through a zone containing a dust'bearinggas in order to obtain vsubstantially-spherical granules; In allof thearrangements described-a pririiaryl source `of vairis introduceddirectly into the' bottomzrrit ofthe system. VSecondary, and eventain'a` liuidizedfb'ed o f dusfttparticles. j

tertiary, aisour'ces lare employed,- particularly to'mainbuilclupnpon*the 'walls ofthle tower directly above or in thevicinity of the exit. Then, as the prills stick to the tower walls, theyplug the tower completely above the exit and the tower must be taken outof service for clean- A fluidzed bed requires a tremendous amount ofpower for the fluidizing air Itiow required to maintain such a bed.Fluctuation of air supply will lead to collapse of the bed.

It has been found that by operating a prilling tower bottom according tothe process described further below, all the drawbacks which areinherent to the prior art described above are greatly reduced, if notcompletely eliminated.

SUMMARY OF THE INVENTION In accordance with the present invention, thereis provided a process for prilling urea and other substances. Withregard to urea, molten substantially anhydrous urea is sprayed and fallsfreely downwardly through a solidification zone in the form of dropletswhich are contacted with a substantially inert and relatively cooler gaspassed into said zone. The droplets are cooled thereby and are solidiedas substantially anhydrous prills. The prills are collected in acollecting through located at the bottom of the solidication zone. A gasis passed through the collecting trough countercurrently to thedownwardly owing prills, the collecting trough comprising an invertedcone or pyramid the apex of which denes an open discharge zone and thesides of which are perforated and oblique and which allow said gas topass therethrough as a iine dispersion to the solidiication zone. Theamount of gas is sucient to divert the downwardly 'iiowing prills fromtheir substantially vertical path to the discharge zone to substantiallyprevent the downwardly iiowing prills from contacting the perforatedoblique sides, and to avoid build-up of the prills upon said perforatedoblique sides.

DRAWING The figure is an elevational view of a prilling towerillustrative of the invention.

SPECIFIC EMBODIMENT OF THE INVENTION Referring to the drawing, stream 1containing substantially molten urea Iat about 280 F. is passed throughspray heads 2, located in top section 3 of prilling tower 4. The moltenurea sprayed into a stream of small liquid droplets through spray heads2 is allowed to fall freely inside the prilling tower 4 countercurrentlyto an uprisingA stream of ambient air introduced into the prilling towerat its bottom section 5. This ambient air, introduced into the prillingtower through openings 6 and 7, in its upwardly :Elow picks up heat fromthe molten urea which crystallizes in the shape of small sphericalparticles (prills). The hot solid urea prills are further cooled toabout 1020 F. above the ambient temperature of the air in openings 6 and7 during their free fall inside the tower 4.

The ambient air introduced into the prilling tower 4 is heated up by theequivalent amount of heat released by the urea prills during their freefall, and it is discharged from the prilling tower 4 through openings 8located in or near top section 3.

The bottom section of prilling tower 4 is provided with a hollowcollecting trough 9 in the form of an inverted cone or frustum withsides 10 made of suitable material as concrete or metal. The sides 10 ofthe inverted trough 9 are solid and are at an approximate angle of about20-30 with the horizontal, and they are covered from the inside withtine wire mesh'll, which is held in place by means of spacers 12. Thewire mesh is provided to disperse the air which is passed therethrough;it has little surface area. such that product cannot collect thereon.The surface area of the solid portion of the minimal. Spacers 12 arepositioned in such a manner as to maintain the wire mesh cover 11 at anapproximate distance of about l-2 inches from the sides 10.

Free space 13 between the wire mesh 11 and the sides 10 is sealed at itsends 14 and 15, respectively, so as to form a sealed chamber all aroundthe total surface of the sides 10. Further, vertical Walls 16 and flatbottom 17 of the bottom section 5 of prilling tower 4 form a sealedchamber 18 with the sides 10 of the inverted trough 9. As shown in thispreferred embodiment, a minor portion, from about 5 t0 about 30 percentby volume, of the total amount of ambient air fed to the prilling tower4 is passed into sealed chamber 18 through line 7 and thence throughopenings 19 provided on the sides 10 into space 13 between the Wire mesh11 and the sides 1t). It is to be understood that all of the airemployed can be fed through line 7. The ambient air of line 7 is evenlydistributed along the whole surface of the trough 9 by being passedthrough the wire mesh 11 and is finely dispersed as it passes throughthe wire mesh. As it leaves the wire mes-h surface, the air issui'licient to divert the free falling solid urea prills from theirvertical path to an oblique path which is converging toward bottomopening 20, located at the apex of the inverted trough 9. Thus,substantially all of the prills do not impinge upon mesh 11, but arediverted toward bottom opening 20. This prevents undesirable build up ofthe prills on the surface -of mesh 11, which would impede upward air owand reduce the etiiciency of tower operation. Due to the fact that thereis practically no upward air ow in the section just above the bottomopening 20, the solid urea prilled product falls through the bottomopening 20 onto belt conveyor 21, from which it is discharged at 22ready for storage or bagging. As indicated, air is not introduced intobottom opening 20 countercurrent to the product passing therethrough. Arubber or other iexible skirt (not shown) can be located around thebottom opening 20 to seal the bottom product discharge area, and tominimize leakage of air from solidication zone 23.

Urea dust present in the tower is removed through bottom opening 20 andcan be separated from urea prills as undersized material in a commercialshaker or the like (not shown). In contrast to prior art towers, thedust is not blown upwardly and out of the tower. Thus, pollutionproblems are minimized with the method of this invention.

In this preferred embodiment, a major portion, from about to about 70percent by volume, of the ambient air fed to the prilling tower 4 isintroduced into the bottom section 5 through openings 6 located aboveand preferably just above the inverted trough 9. This air is mixed withthe smaller portion of the ambient air introduced into the prillingtower 4 through line 7; the total mixture of the two streams of ambientair rises through the prilling tower 4 and it is exhausted from the topsection 3 of the prilling tower 4 through openings 8.

Since the urea droplets are solidified in tower 4, the portion of thetower below spray heads 2 and above trough 9 is considered to comprisesolidilication zone 23.

In another embodiment, the major portion of ambient air can beintroduced into the upper section 3 of the prilling tower 4 through theopenings 8, passed downwardly and co-currently to the stream of freefalling product and exhausted from the prilling tower 4 through theopenings 6 on the bottom section 5 of the prilling tower 4 together withthe minor portion of air introduced into the prill collecting invertedtrough 9 through openings and line 7.

EXAMPLE Referring to the figure, a stream of 25,000 lbs/hr. of moltenurea, at about 280 F., containing about 0.2 weight percent of water andabout 0.3 weight percent of biuret, is passed through line 1 and issprayed through a system of spray heads 2 inside a prilling tower 4having a circular cross-section about 25 feet in diameter. The nedroplets of molten urea thus formed are allowed to fall freely insidethe prilling tower 4 for its full height of about 140 feetcountercurrently to a stream of uprising air, which is exhausted atabout 95 F. to the atmosphere through openings 8 located in the uppersection 3 of the prilling tower 4.

The total amount of air exhausted to the atmosphere through the openings8 is an amount of about 200,000 standard cubic feet per minute(s.c.f.m.), of which amount about 170,000 s.c.f.m. is introduced intothe bottom section 5 of the prilling tower 4 through lines 6 at about 70F.

By the time the free falling urea particles reach the bottom section 5of prilling tower 4 they are solidified into the shape of small spheresof about 1.5 mm. in diameter and are cooled to about 100 F.

About 30,000 s.c.f.m. of ambient air at about 70 F is introduced intothe sealed chamber 18 of the prill collecting inverted trough 9; throughopenings 19, the air is passed through a 50 mesh Wire cloth 11 coveringthe sides 10. This amount of ambient air forced through the wire mesh 11at a relatively high velocity diverts the free falling urea prills fromtheir vertical path into an oblique path converging toward the bottomopening (circular) 20 of about 3 feet in diameter located in the centerof the tower cross-section.

The solid urea prills at about 100 F. are discharged through bottomopening 20 onto belt conveyor 21 and are delivered to storage or baggingat 22. Stream 22 consists of 25,000 lbs./hr. of solid prilled ureacontaining about 0.2 weight percent of water and about 0.3 weightpercent of biuret.

Air is shown as a suitable gas for cooling and diverting free fallingsolid material in tower 4. It is to be understood that any gassubstantially inert to the solid material, as urea, can be employed. Inaddition to dry or humid air, or in admixture therewith, there can beused: nitrogen and carbon dioxide. SO2 and S03 can also be employed withurea, some sulfur or sulfur compound forming on urea prills as a coatingtherefor. Steam is not suitable, since it would increase the moisturecontent in the prilled product.

Instead of forming urea prills, the process can be used to form andcollect such fertilizer substances as ammonium nitrate, ammoniumphosphate, calcium nitrate, calcium phosphate, potassium chloride, etc.,from molten streams thereof charged to tower 4 from line 1. A mixture oftwo or more of such substances, including urea, can be so chargedthrough line 1.

Mesh 11 can be replaced by other suitable elements which provide desiredfine air dispersions. A substitute can be, for example, a perforatedplate preferably having a total opening area larger than plate surfacearea.

I claim:

1. In a process for prilling urea, comprising:

(a) spraying molten, substantially anhydrous urea downwardly as dropletsin a prilling tower through a solidication zone therein generally towardthe bottom of the tower;

(b) contacting said droplets with a substantially inert gas passed intosaid zone to cool and solidify said droplets as anhydrous prills; and

(c) collecting said prills at a centrally located bottom of said zone; l

the improvement comprising:

(d) passing a minor amount, from about 5 to about 30 percent by volume,of said inert gas through a hollow, perforated, inverted frustum trough,and passing the balance of said inert gas into said solidication zoneabove said trough, all of said gas being passed countercurrently to saiddownwardly falling prills, and maintaining gas flow, gas velocity andthe angle of said trough through which said gas is so passed as a nedispersion, so as to divert said downwardly flowing prills from asubstantially vertical path to an oblique path toward said bottom ofsaid zone and to prevent substantially all of said prills fromcontacting said trough.

2. The process of Claim 1, wherein said inert gas is air.

3. In a process for producing substantially spherical fertilizerparticles, comprising:

(a) spraying molten, substantially anhydrous fertilizer in finelydivided particle form downwardly in a tower through a solidificationzone generally toward the bottom of said tower;

-(b) contacting said particles with a substantially inert gas passedinto said zone to cool and solidify said particles; and

(c) collecting the resulting cooled and solidified particles at acentrally located bottom of said zone;

the improvement comprising:

(d) passing a lminor portion, from about 5 to about 30 percent byvolume, of said inert gas through a hollow, perforated, inverted frustumtrough, and passing the balance of said inert gas into said solidicationzone above said trough, all of said gas being passed countercurrently tosaid downwardly falling cooled and solidified particles, and maintaininggas flow, gas velocity and the angle of said trough through which saidgas is so passed as a line dispersion, so as to divert said downwardlyflowing cooled and solidiiied particles from a substantially verticalpath to an oblique path toward said bottom of said zone and to preventsubstantially all of said cooled and solidified particles fromcontacting said trough.

4. The process of Claim 3, wherein said inert gas is air.

References Cited UNITED STATES PATENTS 3,615,142 10/1971 Dahlbom 264-133,457,336 7/1969 Harris 264-14 ROBERT F. WHITE, Primary Examiner J. R.HALL, Assistant Examiner

